Photoconductive type pickup tubes



Aug. 1s, 1959 A. D. COPE 2,900,569

PHOTOCONDUCTIVE TYPE PICKUP TUBES i Filed July l1, 1955 INVENTOR.

Unite Appleton D. Cope, Hightstown, NJ., 'assigner to Radio Corporationof America, a corporation of Delaware Application July 11, 1955, SerialNo. 521,172

7 Claims. (Cl. 313-65) This invention relates to transducing electrontubes, and more especially it relates to such tubes of theelectrooptical kind for converting optical images into correspondingelectric signals.

A principal object is to provide an improved electron tube of the kindemploying a light-responsive variable conductivity target electrode fortranslating light values at elemental areas of the target intocorresponding electric signals.

Another object is to provide an improved photoconductive targetelectrode for stabilizing the operating characteristics of televisionpickup, or camera tubes.

A feature of the invention relates to an improved target electrode forelectron tubes wherein the target includes an element which is normallyof high electrical insulation when not exposed to light, but which iscapable of assuming a potential at each of its elemental areas tocorrespond with the light intensities incident thereon, and is alsocapable of being restored to a predetermined datum or base potential ateach area when scanned by an electron beam.

Another feature relates to an electro-optical transducing target of thelight-responsive variably conductive kind arranged to be scanned over apredetermined scanning region, or raster, which may be of differentshape and size from the shape and size of the layer of photoconductivematerial, in conjunction with a conductive backing or signal plate whoseshape and size are closely correlated with the shape and size of theraster, thereby avoiding certain instabilities and non-uniformities ofoperation which may occur with conventional targets of thephotoconductve type.

A further feature relates to a target electrode for pickup tubes and thelike, wherein there is employed a composite transducing elementincluding a highly conductive lighttransparent backing or signal platesupported on one end of the tube or face plate thereof without entirelycovering the face plate; and the face plate is covered with a layer ofphotoconductive material which extends substantially beyond the edges ofthe signal plate, the size of the signal plate and its shape beingclosely correlated with the size and shape of the scanning raster.

VA still further feature relates to the novel organization, arrangementand relative location and dimensioning of parts, which togetherconstitute an improved target or pickup tubes of the charge storagetype.

Other features and advantages will be apparent from the ensuingdescriptions and the appended claims.

In the drawing, which shows a typical example of one embodiment,

Fig. l is a sectional view of an electro-optical transducing tube inaccordance with this invention;

Fig. 2 is an enlarged plan view taken along the line 2--2 of Figure l;and

Fig. 3 is a sectional. view of Fig. 2.

The invention finds its primary utility in television pickup, or cameratubes, of the type having a photoconductive target electrode. Pickup, ofcamera tubes of this j 2,900,569 Patented Aug. 18, 1959 type generallycomprise an evacuated bulb within which is mounted an electron gun ofany well known construction for developing a deectable scanning beam ofelectrons usually referred to as a cathode-ray beam. The beam is focusedon, and deliected over, a photoconductive transducing target toconstitute a raster of well deiined size and shape, usually square orrectangular, i.e. having a conventional four-by three aspect ratio. Thetarget is usually supported on a light-transparent backing, which forexample may constitute :the end wall or face plate of the glass bulb.Such a target usually consists of a light transparent layer or lm ofhigh electric conductivity which is coated or otherwise applied to thesubstantially circular, inner surface ofthe glass support facing theelectron gun, and then a layer of photoconductive material, such forexample as antimony tri-sulphide, is applied over the conductive layer.The photoconductive material is substantially an insulator when notexposed to light rays. The target hasrthe property of coacting mutuallywith the scanning beam and with the light intensities existent on eachsuccessive elemental area of the target so as to cause the elementalareas to change their conductivity and thereby produce signal voltagesunder control of the scanning beam.

The photoconductive layer of the target, whether it be antimonytri-sulphide, or any other photoconductive material, being anon-conductor when not exposed to light, nevertheless has itsconductivity at each elemental area thereof variable in accordance withthe variations of the light intensity to which each such area isexposed. In other words, the conductivity of the photoconductive layeris proportional to the amount of light exciting it and the variation islimited to the particular areas which are so excited. Thus, when thesurface of such material is subjected to light from a scene or subjectto be televised, the various elemental areas of the target change theirnormal non-conductance condition to correspond respectively with thelight intensities from respective elemental areas of the scene orsubject being televised, it being understood, of course, that theselight rays are suitably projected or focused on the target.

Usually the transparent conductive backing or signal plate on which thephotoconductive material is applied, is biased to a steady positivetarget potential, e.g. from 10 to 200 volts, with respect to the cathodeof the electron gun which scans the target. In eifect, the conductivesignal plate and the photoconductive coating thereon constitute acharge-storing condenser with the charge at each elemental area being afunction of the incident light between scannings by the electron beam.The signal currents, or impulses, are produced by subjecting thecondenser to a discharging action by the scanning beam which restoreseach elemental area that it strikes to a predetermined datum potentialfor example cathode or nearcathode potential. It is clear, therefore,that if any elemental areas of the target have received a charge, eitherby dark conductivity or by exposure to light, and have not beenappropriately discharged by the scanning beam, they tend to assume anundesired steady potential. I have found that this effect tends toproduce unstable electric signal output characteristics, especiallywhere the area that is not properly discharged is not located within theboundaries of the actual scanning raster.

Usually, the light transparent conductive coating or signal plate isapplied over substantially lthe entire end wall or face plate of thetube so as to make complete peripheral contact with a separateconductive ring sealed through the tube wall and by means of which thepositive biasing potential can be applied thereto; and usually thephotoconductive material is substantially coextensive with theconductive signal plate. Since the face plate is usually circular andsince the raster is usually square 3 or rectangular, there alwaysexistsa region surrounding the raster which is not subjected to the dischargeor cathode stabilization by the cathode-ray beam.

I vhave -found that this area beyond the boundariesof the raster, aftera period of operation -of the tube,acquires a positive potential whichis substantially demarcated in level from the potential level of thescanned raster area. The scanned raster area over a period of repeatedscannings `can be considered as having ground or near-ground potential.In other words, there is a well defined potential step between thescanned and nonscanned areas of the photoconductive layer. I havefurther found that the area of undesired positive potential beyond theboundaries of the raster tends to spread its effect into the rasterarea. The net result is that the edges of the picture reproduced undercontrol ofthe signals from such a conventional pickup tub' target,'tendto show crawL ripple or other instability. In extreme cases thecathode-ray beam is actually unable properly to discharge the targetadjacent the edges of the raster so that the ytarget becomes inoperativeas a proper transducing agent at the edges of the raster.

The present invention, therefore, has for one of its principal objects,overcoming of the above noted and other disadvantages. Referring to thedrawing, the tube may comprise an evacuated glass bulb, or envelope,which has suitably supported at one end therein an electron Vgun 11 ofany well' known construction comprising, for example, an electronemitting cathode 12, a control electrode 20 and one or more acceleratingelectrodes 22 for developing an electron beam which can be focused in anelemental spot upon the light transducing target electrode 13 mountedwithin the opposite end of the bulb. This target electrode comprises alight transparent backing iilm 14 of a good electrical conductor such,for example, as a tin compound like tin oxide or tin chloride. Theconductive film 14 can be applied in any suitable manner, for example asa coating, directly on the inner surface of the light transparent endwall or face plate 15 of the glass bulb.

In accordance with the invention, the conductive light transparent lm14, which constitutes the signal plate, does not completely cover theend wall or face plate of the tube but is of square or rectangular shapeand provided with at least one laterally extending portion 16 for makingContact with a metal ring 17 sealed into and through the wall of theglass bulb. It will be understood, of course, that the metal ring 17should be of a suitable metal or alloy which has substantially the samecoeil'- cient of expansion as the glass of the bulb 10 and of the faceplate 15 so'as to preserve a vacuum tight seal. If desired, one or moresimilar narrow strip extensionsrl of the lm 14 may be provided toincrease the contact area between the ring 17 and the signal plate 14.The size and shape of the raster 19 scanned by the electron beam areindicated in Figure 2 by the dot-dash outlfne.

In accordance with another feature of the invention, the size of therectangular or square conductive signal plate or iilm 14 is onlyslightly less than the actual size and shape of the scanning raster 19.

After the conductive iilm 14, with its integral connection strips 16 and18, has been applied to the face plate 15, a layer of photoconductivematerial23 is applied over substantially the entire inner surface of theface plate 15. The invention is not limited to any particular manner ofapplying the lilm 14 and the photoconductive material 23. For example,the photoconductive material 23 may be applied by any of the knownevaporation techniques. Of course other methods of applying thematerials may be utilized in accordance with this invention. The layer14 may be any transparent, conductiveV material, e.g. tin chloride, and'may be applied by any of the known techniques. After the materials 14and 23 have been applied, the tube, is subjected to the usual .i2,900,569V f jr evacuation and other techniques well known in theelecftron tube manufacturing art.

When the tube is to operate, for example, with a low velocity electronbeam, the ring 17 can be connected through a suitable load resistor 21to a source of positive direct current potential, whereby film 14 isbiased to as much as 200 volts positive with respect to cathode 12whichV may be grounded. The usual conductive Wall coating, or nal`accelerating electrode 24, can 'be connected to a source of positivepotential, for example positive 250 or 300 volts, as is the final anodeof the gun 11. The electron beam, therefore, deposits upon eachelemental area of the target suiicientv electrons to neutralize theaccumulated positive charge at each area resulting from its lightexcitation, and in doing so there are generated the proper video outputsignals across resistor 21V which can be coupled to any well known videoamplilier (not shown).

Y, over said conductive layer and facing said means, and saidphotoconductive layer extending beyond the *boum* l have found that,with the substantial correlation between the size and shape of thesignal plate :14V and the actual scanning raster 19, the above notedVinstability at the edge of the raster is avoided and the tube is stablein its output characteristics. i

lt will be understood, of course, that the invention is not limited tothe particular materials mentioned forthe conducting iilm 14 and thephotoconductive material 23. Other photoconductive materials, such ascadmium-sulphide or selenium, may be employed.

While the invention has been described in connection with a square orrectangular signal plate 14 and a corresponding raster, it will beunderstood that if the raster is of any other shape, for examplecircular, then likewise the signal plate 14 should also be circular andof substantially the same size and shape as the raster. thermore, whilereference has been made to the operation of the tube with aV scanningbeam of low electron velocity, it is believed that the invention alsocan be used with the tube operating with a scanning beam of high'Yelectron velocity, which causes the target to emit sec ondary electronswhen struck by the scanning beam, in which event a suitable secondaryelectron collector grid may be mounted adjacent to the target on theside facing the gun, as is well known in the art. It will be understood,of course, that any well known means may be provided for focusing thebeam from the electron gun in an elemental spot upon the target 13, andthat any Well known means may be employed for subjecting the beam to thedesired scanning pattern to cover the desired raster.

. Various changes and modifications may Vbe made in the disclosurewithout departing from the spirit and scope of the invention.

What is claimed is:

1. Electron tube apparatus comprising, a tube having means to develop abeam of electrons, a light-responsive` target arranged to be scanned bysaid beam ina scanning raster for developing output electric signalscorrespond# ing to the light excitation of the scanned areas of thetarget, said target including a conductive layer of a predeterminedsize, means for making an electrical connection with said conductivelayer, a photoconductive layer daries of said conductive layer.

2. Electron tube apparatus comprising, a tube having an electron gun fordeveloping Va beam of electrons, a light-responsive layer ofphotoconductive material arranged to be scanned by said beam in arasterof predetermined shape and size todevelop output electric sig-Ynals corresponding to the light excitation of the scanned areas of thetarget, saidhtarget comprisingV a layer of photoconductive material'ofthe kind which is normally a non-conductor when not exposed to light butwhich becomes a conductor of variable conductivity corresponding to thevariation of light thereon, said layer extending substantially beyondthe boundaries of said raster, a light-transparent metal lm in directContact with said layer on the side of said layer remote from the gun,said ihn having a size slightly less than said raster, and lead-in meansto apply a biasing potential to said lm.

3. Electron tube apparatus comprising, an evacuated envelope having anelectron gun to develop a beam of electrons, a light-transparentsupport, a light-responsive target of the photoconductor kind carried bysaid support and arranged to be scanned in a raster of a predeterminedsize, an electrically conducting signal plate atfrached to said supportand having a size which is slightly smaller than the size of saidraster, and a coating of photoconductive material covering said signalplate on the side thereof facing the gun and substantially covering thesurface of said support beyond the boundaries of said signal plate.

4. An electron discharge device especially designed for transducinglight signals into corresponding electric signals, comprising anevacuated bulb having a conductive lm coated on a wall of the tube, acoating of photoconductive material on said film, said conductive lmbeing located substantially within the boundaries of said coating toleave a portion of said coating which is unbacked by said film, aconductive ring surrounding said coating and electrically connected tosaid conductive lrn, an electron gun, and means for applying potentialsto said device for scanning said layer with electrons from said gun in araster, said raster being slightly larger in size than said conductivefilm.

5. A pickup tube comprising an evacuated bulb having a face plate at oneend, a layer of photoconductive material substantially coextensive withsaid face plate, a light-transparent signal plate sandwiched betweensaid layer of photoconductive material and said face plate and having asurface extent which is substantially less than said face plate, and anelectron gun for scanning said layer of photoconductive material in apredetermined raster, said raster being slightly larger than said signalplate.

6. A pickup tube comprising an envelope, means for producing an electronbeam in one end of said envelope, a target electrode in the other end ofsaid envelope and in the path of said beam, means for scanning said beamover said target in a raster pattern, said target including aphotoconductive layer substantially larger than said raster pattern anda signal plate, said signal plate being slightly smaller than saidraster pattern whereby charges at the edges of said raster aredischarged by said beam.

7. An image transducing tube of the kind described comprising, anevacuated bulb, a light-transparent support, a photoconductive targetattached to said support, said targetk including a layer of normallynon-conducting photoconductive material, a light-transparent conductivesignal plate sandwiched between said layer and said support, said platehaving a surface extent which is substantially less than that of saidlayer, an electron gun for scanning said target with an electron beam ina raster which is only slightly larger in surface extent than saidsignal plate but is substantially smaller in surface extent than saidlayer, and lead-in means between said support and said photoconductivematerial for applying a biasing potential to said signal plate.

References Cited in the le of this patent UNTED STATES PATENTS 2,368,884Shade Feb. 6, 1945 2,654,852 Goodrich Oct. 6, 1953 2,687,484 Weimer Aug.24, 1954

